TOKYO (AP) An Air China passenger jet was forced to return to Japan and four others were delayed Friday after a bomb threat was e-mailed to the airline's Tokyo office, the Japanese Transport Ministry said.

The anonymous e-mail, written in Japanese, told the Chinese airline to suspend its flights or the writer would "bomb the aircraft," ministry official Fumio Yasukawa said.

"We suspect this is a threat related to the Olympics," he said, refusing to provide further details of the note or say whether any particular groups were suspected of sending it. The opening ceremony for the Beijing Olympics was held later Friday.

China's official Xinhua News Agency said the airline ordered all of its flights not to depart from Japan until their safety was ensured.

The plane that was forced to return to Japan, which was carrying 70 people from Nagoya to Chongqing via Shanghai, and four other delayed Air China flights took off later Friday after no bombs were found on them, ministry officials said.

Police are investigating the case but don't have any suspects, a Tokyo Metropolitan Police official said on condition of anonymity, citing department policy.

Another Transport Ministry official, Morihiro Kawada, said aviation officials had suspected the e-mail was a hoax, but "it is a time when we must be extra careful about safety and we should not draw easy conclusions."

Transport officials said the bomb threat did not affect Japanese safety measures.

"We have already instructed all airlines flying between Japan and China to carry out maximum safety checks during the Olympics," Kawada said.